Carrier transport through photo-gated thin exfoliated dichalcogenide films

Abstract

Transition-metal dichalcogenides thin crystals rise as potential building blocks of thin electro-optical devices. Their electronic structure can be locally modified applying local stress or local electrostatic potentials (gates) to induce the appearance of barriers and/or wells for carrier confinement. Given the feasible manipulation of these quasi-2D systems, they can be used as thin film switches with advantage if compared to analogous bulk semiconductor devices which are rigid, heavy in weight, and demanding high cost technology for processing. They hold the promise for the realization of new generation of 2D storage devices, solar cells, photodiodes, and light-emitting diodes. Within the scope of this theme, a challenging problem has been proposed: the characterization of transport effects on photo-gated MoS2 thin films with functionalized substrate using photochromic molecules. This can be achieved by placing the exfoliated crystal on top of photochromic self-assembled azobenzene molecules, thus being able to control the potential profile topography by optical means. The molecular photo-gating is achievable through the trans to cis isomerization of the azobenzene molecules and can be reversed by suitable illumination.These molecules, in turn, can be chemically tailored to be n- or p-doped. Understanding the nature of the effects controlling the diversified transport response in this novel 2D systems and foreseeing the optimization of their functionalities have become strategic tasks. This has been a process that has involved both experimental and theoretical endeavors. The main theoretical hypothesis assumed to tackle this problem is that the transport response is driven by the elastic scattering of electrons against the induced topographic structures in the 2D potential profile controlled by the substrate and the objectives of this project are: study of the properties that arise from the MoS2 interaction with substrates, simulate the potential profiles for the available experimental conditions, simulate the current response under various voltage configurations, elucidate the role of spin-orbit coupling on the possible appearance of spin-Hall effect as gates/potentials are implemented. The steps to achieve the proposed objectives will allow: (i) broadening the student knowledge about the effects of the physics of molecular adsorption; (ii) enhancing electronic structure calculation tools to be adapted to the proposed problems; (iii) contribute to the understanding of scattering modulation experimentally detected in samples with special geometries (in collaboration with experimental partners); (iv) seeking, whenever possible, the correlation between theory and experiment, indicating potential problems of common interest, proposing experimental settings and systems where new material effects can be revealed as well as the conditions to ensure their observation: dependence on the substrates and control of geometrical parameters of the potential profiles. (AU)